The present invention is directed to a system for sensing lateral displacement and, more particularly, relates to a system for precisely measuring lateral displacement of a body without requiring contact thereto and lateral extension of a body, also without requiring contact thereto.
There is a well known need in the prior art for devices that accurately measure lateral extension of an object. Typical of that need is in the tensile testing area where precise measurements of changes in length of specimens (metal, ceramic, etc.) under application or removal of a force are required. Extensometers are instruments conventionally used to provide the desired lateral displacement information. The accuracy and precision of extensometers, however, is limited. Moreover, the necessity for direct contact between the extensometer and the object undergoing measurement prevents application in high temperature and corrosive environments and areas of limited access.
An apparatus for interferometrically detecting lateral motion, shown in FIG. 1, utilizes two laser beams, derived from the same laser. The beams are incident at an angle .theta. at a point P on a scattering surface. In the region in which two beams overlap, an interference fringe pattern is formed as shown. The fringe spacing d is given by: ##EQU1## where w is the wavelength of the laser. When the surface moves, particles on its surface pass through the spacially modulated intensity pattern and produce a temporally modulated scattered intensity with a frequency f: EQU f=v/d=2vsin.theta./w,
where v is the velocity component in the plane of the surface and perpendicular to the interference fringes. By detecting this intensity modulation, the lateral velocity of the surface can be monitored, and its displacement can be measured by integrating the velocity (dx=vdt).
The apparatus described above for interferometrically detecting lateral motion suffers from several defects. In particular, low frequency intensity modulations of the laser- or of the detector current--can be misinterpreted as surface displacements. The apparatus can also not sense the direction of the motion.
It is therefore a principal object of the present invention to provide an apparatus for precisely measuring lateral displacement of a surface.
It is a further object of the present invention to provide an apparatus for precisely measuring lateral displacement of a surface without directly contacting the surface.
Yet another object of the present invention is to provide a method of detecting lateral displacement which can sense the direction of the motion and is highly reliably and sensitive. Furthermore, excellent measurement results may be achieved even when the surface whose extension is to be measured is unprepared or has a diffused surface. A grating on the surface to be measured is not necessary with the present invention. In practice, it may be impractical to affix a grating to the surface whose extension is to be measured in many environments.